Method for fabricating optical devices in photonic crystal structures

ABSTRACT

A method for manufacturing optical components in a three-dimensional photonic crystal lattice. A first resist ( 9 ) is coated on a substrate ( 10 ) and exposed to an e-beam ( 11 ), to produce an imaged area ( 12 ). Another resist coating is applied to thicken the resist ( 13 ) and an interference exposure ( 15 ) is used to image the result. This is developed to form periodic voids ( 16 ), which may be filled with a materials having a high refractive index to form a pattern ( 18  and  12 ) when the resist ( 13 ) is removed.

BACKGROUND OF THE INVENTION

The present invention relates to integrated optical circuits, which arefabricated in a photonic crystal. Specifically, a method is disclosedwhich creates optical devices in a photonic crystal structure using bothelectron beam (e-beam) and optical exposure of a resist to define anoptical device surrounded by a dielectric periodic structure, a photoniccrystal.

Optical circuits may be fabricated in photonic crystals, which consistsof optical devices surrounded by a dielectric periodic structureexhibiting a photonic bandgap. The photonic bandgap is a range offrequencies which will not propagate through the periodic structure. Theparameters of the dielectric periodic structure include the periodlength, refractive index of the structure, the shape of the periodiclattice as well as other factors, which determine the frequency of lightthat cannot propagate within the periodic structure. Light having afrequency within the photonic bandgap of the dielectric periodicstructure is confined in the optical device, such as a waveguide, whichconstitutes a defect in the periodicity, and its further propagation iscontrolled by the optical device.

Processes for manufacturing such integrated optical circuits aredisclosed in a published U.S. patent application (U.S. 2002/0074307A1),as well as other references. The dielectric periodic structures areformed within the dielectric layer, and an optical device such as awaveguide is formed by creating a longitudinal interruption in theperiodic structure. Light having a frequency within the correspondingbandgap of the material is reflected along the internal surfaces of thelongitudinal structure.

Another way of utilizing photonic crystal for waveguiding is byengineering its dispersion properties so that light is forced topropagate only along certain discrete number of directions. In thiscase, for arbitrary light routing, defects in the form of mirrors can beused to redirect the propagation of light from one allowed direction toanother.

In either case, the usefulness of a device based on photonic crystalsrests on the ability to create defects in the regular periodic array ofthe photonic crystal.

The creation of the periodic dielectric structure and optical componentswithin the periodic structure typically requires the use of photo masksto define features in the integrated circuit corresponding to theperiodic dielectric structure and the optical components. The use ofphoto masks is a significant cost factor in the manufacturing of suchintegrated circuits. Moreover, the creation of three-dimensionalstructures with devices embedded in the photonic crystal matrix requiresa multilayer fabrication. In this case a working device is possible onlywith very precise alignment between layers, which is both costly andtime consuming.

Other techniques for creating the periodic dielectric structure includesthe use of optical interferometric lithography, which is useful forexposing large areas of a dielectric surface, and for creating athree-dimensional periodic dielectric structure. The process eliminatesthe need for a photo mask, however, not all sizes and configurations ofan optical device can be formed within the periodic dielectric structureusing interferometric lithography exclusively. In particular, theplacement of defects, such as waveguides, resonators, or mirrors, isimpractical with interferometric lithography alone.

Electron beam (e-beam) technology permits the creation of very highresolution patterning on a substrate. E-beam exposure also permits awell determined penetration depth to be obtained within a layer ofresist used to form patterns of circuit components on a substrate.However, patterning large surface areas using e-beam exposure isrelatively slow. Moreover, the creation of 3D patterns requiresmultilayer processing with all the drawbacks listed above.

It is an object of the present invention to combine opticalinterferometric lithography for exposing large areas of resist to createa three-dimensional periodic structure with e-beam exposure techniquesto manufacture optical circuits embedded in the three-dimensionalperiodic structure.

BRIEF SUMMARY OF THE INVENTION

A process is provided for manufacturing three-dimensional photoniccrystals using both optical exposure techniques as well as electron beamexposure techniques. The optical exposure techniques are usedinterferometrically on a resist material to define three-dimensionalperiodic structures. Additionally, the resist material is electron beamsensitive. Using e-beam exposure techniques, the e-beam penetrationdepth in the resist is controlled, and three-dimensional components maybe defined within the resist material having the requiredthree-dimensional geometry. As a result of using both exposure methods,large regions of the photo resist may be exposed using optical exposureto define three-dimensional periodic structures, and smaller regionsconstituting component features may be defined using the e-beam exposureprocess.

In accordance with a preferred embodiment of the invention, thesubstrate is covered with a resist material, which is also sensitive toelectron beam radiation, and component structures are patterned in theresist by e-beam exposure. Following the formation of these patternsrepresenting the three-dimensional optical components, a second layer ofthe resist is deposited. The resist is then exposed to coherentradiation using a volumetric interference pattern of multiple coherentbeams of radiation.

The exposed resist is then chemically developed. For resists havingpositive contrast for optical radiation and negative contrast toelectron beam, such as AZ5214, areas which were exposed to light fromthe interference pattern and unexposed by the electron beam are removed.The result is a pattern consisting of a regular grid of holes,corresponding to antinodes formed by the interference pattern, as wellas the component features representing defects in the interferencepattern which were created by the e-beam. In order to increase thedielectric contrast, the holes are back-filled with a high refractiveindex material, creating a three-dimensional dielectric contrast whichis periodic, surrounding components formed from patterns created usingthe e-beam. The remaining resist is chemically dissolved resulting in athree-dimensional periodic structure with a high dielectric contrast,which includes an optical component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a sectional view of an optical integrated circuit ofphotonic material having a waveguide;

FIG. 2 is a micrograph of a substrate having a resist coating exposedwith an electron beam;

FIG. 3A illustrates the result of variable kV electron beam exposure ofa resist coating on a substrate;

FIG. 3B illustrates a plan view image of a periodic array structurehaving two-dimensional periodicity and a waveguide;

FIG. 4A illustrates the step of coating a substrate with a resist;

FIG. 4B illustrates the step of exposing the resist with an e-beam topattern a three-dimensional component;

FIG. 4C illustrates the component pattern created by exposing the resistwith an e-beam;

FIG. 4D illustrates the step of forming a second layer of resist on thesubstrate;

FIG. 4E illustrates the process of interferometric exposure for creatinga pattern of a periodic dielectric structure in the resist;

FIG. 4F illustrates the process of developing the exposed resist;

FIG. 4G illustrates back filling of voids created by the interferinglight beams with a high dielectric material; and

FIG. 4H illustrates the step of dissolving the resist creating a latticeof dielectric surrounding a waveguide structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a sectional view of a three-dimensionalphotonic crystal with an embedded waveguide 4 is shown. A substrate 1 iscoated with a dielectric 2, and a plurality of holes 3 in the dielectriclayer 2 create a three-dimensional periodic structure which confineslight to propagate along waveguide 4 due to the dielectric contrastpresented by the holes 3 in all directions. An optical component such asa waveguide 4 for light having frequencies within the bandgap is formedby creating a defect in the periodic structure in the dielectric 2. Thelight is confined in all directions within the waveguide as it is guidedalong the inner surfaces of the waveguide structure.

The present invention makes use of two distinctly different processesfor patterning features in the dielectric 2. In the preferredembodiment, the process uses a resist material which has a positivephoto resist characteristic when subject to optical exposure, and whichhas a negative resist characteristic when exposed to an electron beam(e-beam). Optical components such as waveguides, cavities, resonators,switches, etc. may be precisely patterned in the resist using thecontrollable e-beam to expose the resist. Subsequent layers of resistmay thereafter be applied to the surface, and the resist is subject toan optical exposure to create the surrounding 3-dimensional periodicstructure.

Surface features produced from patterns produced by e-beam exposure areshown in the micrograph of FIG. 2. A plurality of 25 pads 8 are createdby exposing the area of pads 8 on a photo resist such as AZ5214 with thee-beam. The photo resist has a positive resist characteristic to UVexposure, but exhibits a negative resist characteristic to e-beamexposure. Following the step of exposing pads 8 to the e-beam, theentire surface is exposed to ultraviolet radiation (UV). The photoresist is then developed using, for instance, an MIF327 developer. Pads8 which were exposed by the e-beam are not dissolved by the developer asa result of the resist having a negative e-beam characteristic.

FIG. 3A illustrates the effect of the e-beam accelerating potential onthe resist. Supporting structures 6 in the resist are produced byselectively irradiating the surface using an e-beam having anaccelerating potential of 20 kV, and connecting bridges 7 are formed byirradiating the area between the supporting structures 6 with an e-beamaccelerating potential of 5 kV. After the e-beam exposure, the entiresurface is flooded with UV radiation. Following developing of theexposed surface, the remaining resist thickness in the connectingbridges 7 have a thickness which is established by the acceleratingpotential of the e-beam used to expose them. As 20 kV electronspenetrate and expose the full thickness of the resist, the height ofsupporting structures is determined by the original resist thickness.

FIG. 3B illustrates the results of combining e-beam exposure techniqueswith UV exposure techniques. The defect 12 represents a waveguidestructure in the periodic structure 18 containing a plurality of holesgenerated by exposure to ultraviolet (UV) radiation. The holes areuniform across a 200 μm square. The waveguide structure 12 results fromthe e-beam exposure which protects the region from the subsequent UVexposure steps. The e-beam process has made it possible to selectivelydesensitize the resist so that the defect 12 or interruption may beformed in the periodic structure 18.

The e-beam process is used in combination with the UV process to formoptical components as well as the 3-dimensional periodic structure byemploying the process steps of FIGS. 4A-4H. The first step of theprocess shown in FIG. 4A is to coat the substrate 10 with a layer of theresist material 9 which has a UV positive resist characteristic and ane-beam negative resist characteristic. FIG. 4A shows the results ofspinning a liquid solution of the AZ5214 resist material 9 on thesubstrate 10 in a liquid solution, and then evaporating the solvent ofthe solution. FIG. 4B illustrates the step of using a controlled e-beam11 to create a pattern representing an optical component such as awaveguide in 3-dimensions in the resist material 9. Depending on theaccelerating voltage of the e-beam electrons exposing the resist 9, thepenetration depth in the resist is controlled so that a defect iscreated having the desired 3-dimensional geometry. Thus, planardimensions in the surface of the resist 9 are controlled as well as thedepth or height of the resulting exposed area to create the pattern 12of an optical component such as a waveguide in predefined 3-dimensions.

Once the pattern 12 has been created by the e-beam for the opticalcomponent, where the e-beam exposed areas are desensitizes as shown inFIG. 4C, another layer of resist 13 is deposited as shown in FIG. 4D.

Now that the pattern for the optical component has been created by thee-beam, the entire photonic crystal can be exposed using interferometricUV exposure as shown in FIG. 4E. The entire volume of resist comprisingnewly deposited layer of resist 13 and resist layer 9 is exposed by aplurality of coherent beams 15 to form a pattern, which contains exposedand unexposed areas resulting from the interference pattern produced bythe sources of radiation.

The exposed resist material 13 is then chemically developed bydissolving the areas which had been exposed to light (antinodes of theinterference pattern) and unexposed to the electron beam as shown inFIG. 4F using a developer such as MIF327. This results in the series ofregular voids or holes 16 forming a grid which corresponds to theantinodes of the interference pattern.

The voids 16 created in the resist layer are filled as shown in FIG. 4Gwith a high refractive index material such as silicon. The highrefractive index material results in a periodic structure 18 having ahigh dielectric contrast which is periodic in 3-dimensions.

The resist is then dissolved in a process step shown in FIG. 4H. The3-dimensional periodic structure 18 exists everywhere except in 12 wherethe e-beam had originally exposed the resist.

Thus, there has been described a process for creating integrated circuitcomponents embedded in a three-dimensional photonic crystal matrix.

While the description above contains many specifics, they should bedeemed for illustration only. Many modifications can be made by thoseskilled in art that are within the spirit of the invention. Inparticular, it is obvious that the resist, AZ5214, can be replaced by anumber of others. The only requirement on the resist is that it issensitive to both optical radiation and electron beam radiation. Whetherthe resist exhibits positive or negative contrast to either radiation isalso immaterial as long as areas exposed to e-beam are sufficientlymodified before the subsequent optical exposure so as to produce defectsin the regular lattice formed by nodes and antinodes of theinterferometric exposure. In addition, the number of layers is notlimited to two, but can be extended as desired for a particularapplication. Moreover, since the substrate is only for mechanicalsupport, its presence is optional as long as there exist other means ofmechanical support. Therefore the description above should not beconstrued as limiting the scope of the invention, which is determined bythe appended claims.

1. A process for forming optical integrated circuits comprising:depositing a resist layer on a substrate having a positive resistcharacteristic to optical radiation and negative resist characteristicto an electron beam; forming optical structures in said resist byselectively exposing said resist with and electron beam; and forming aperiodic structure outside of said optical structures which exhibits aphotonic bandgap.
 2. The process for forming optical integrated circuitsaccording to claim 1, wherein said periodic structure is formed by:exposing said resist with an interference pattern of optical radiation,filling voids created in said interference pattern with a dielectricmaterial; and removing said resist material to create a periodicstructure having a photonic bandwidth.
 3. The process for formingoptical integrated circuits according to claim 1, wherein said opticalstructures are three dimensional with one of said dimensions beingcontrolled by selecting a predetermined electron beam potential.
 4. Theprocess for forming optical integrated circuits according to claim 3,wherein said electron beam potential is selected to 0.5-20 kV.
 5. Theprocess for forming optical integrated circuits according to claim 1,wherein said optical structure is a waveguide.
 6. A process for formingoptical integrated circuits comprising: depositing a first resist whichhas a positive resist characteristic to ultraviolet light and a negativeresist characteristic to an electron beam on a substrate; creating apattern for defining an optical component on said resist by exposingsaid resist with an electron beam having an accelerating potentialselected to control one dimension of said component; depositing a secondlayer of resist on said first resist layer following exposure of saidfirst layer of resist by said electron beam; exposing said layers ofresist with multiple interfering light beams forming an interferencepatter in the resist layers; developing said layers of resist to createvoids in locations exposed by said interfering light beams; backfillingsaid voids with a material having a high index of refraction; anddissolving said resist to produce a three dimensional periodicdielectric with a high index of refraction contrast.
 7. The process forforming an optical integrated circuit according to claim 6, wherein saidoptical component is a waveguide.
 8. The process for forming an opticalintegrated circuit according to claim 7, wherein resist is patternedusing said electron beam to form one of an optical cavity resonator orswitch.
 9. A method for creating a latent image of a periodic structurewith embedded defects comprising: preparing a first layer of resist,said resist being sensitive to both optical radiation and electron beamradiation; selectively exposing said first layer of resist with anelectron beam in an imagewise fashion, where the electrons of saidelectron beam move at a velocity required to penetrate only a predefinedthickness of the outmost regions of said layer of resist; depositing asecond layer of resist sensitive to optical radiation; exposing theresist multilayer to a plurality of coherent beams of optical radiationthat form an array of nodes and antinodes corresponding to aninterference pattern in the volume of said resist multilayer; wherebysaid latent image of a periodic structure is formed in the volume of theresist multilayer created by said array of nodes and antinodes, whichalso contains said embedded defects created by said electron beamradiation.
 10. A method of claim 9, where said layer of resist isprepared by depositing its liquid form on a substrate and evaporating asolvent.
 11. A method of claim 9, where said resist has a negativecontrast for said electron beam exposure.
 12. A method for creatingperiodic structure with embedded defect comprising a method for creatinga latent image of a periodic structure of claim 9 followed by chemicaldevelopment process where portions of the resists are selectivelydissolved depending on their exposure.
 13. A method for creatingperiodic structure with embedded defect in a material comprising amethod for creating periodic structure with embedded defect of claim 12followed by backfilling the voids with said material.
 14. A method ofclaim 13 followed by removing said resist.